1. Field of the Invention
The present invention relates generally to heat exchange devices and methods of performing chemical processes using heat exchangers.
2. Discussion of the Background
Chemical processing systems combining heat exchangers and catalytic reactors are well-known in the art. Significant progress has been made in the field of single assemblies that combine heat exchange and reaction functions due to an increased sensitivity to mechanical equipment size and cost. An example of this trend is the advanced hydrogen generating reactor disclosed in U.S. Pat. No. 6,497,856 to Lomax et al., which combines several heat exchangers and reactors into a single mechanical device. Such combined reactors have been advantageously applied to hydrogen generation for fuel cells, although many other applications are possible.
In most catalytic reactors, reaction rates are extremely sensitive to temperature. In some reactions, the actual product distribution and reaction route can also be profoundly affected by small swings in temperature. One problem encountered whenever a large heat exchange array is integrated with a large adiabatic reactor, such as a packed bed or monolithic reactor, is the presence of temperature gradients across the catalyst bed. These temperature gradients necessarily arise in any cross-flow heat exchange structure, such as a baffled tubular heat exchanger or a plate-fin heat exchanger. In traditional systems using separate heat exchangers and reactors, the fluids of different temperatures would be mixed after heat exchange and before being piped to the subsequent reactor. Accordingly, traditional systems did not encounter concerns regarding temperature gradients. However, these systems required more complicated, less compact, heavier equipment with high heat losses as compared to an integrated reactor and heat exchanger.
Referring to FIG. 4, the reactor of the Lomax et al. patent has an inlet for mixed, pre-vaporized fuel and steam 101, which communicates with a plenum 102, which distributes the mixture to the array of reactor tubes 103. The reactor tubes 103 are provided, as is illustrated in the cut-away portion of FIG. 4, with a charge of steam reforming catalyst material 105. This catalyst material 105 may be a loose packing as illustrated, or may be a catalytic coating, or may be a section of monolithically-supported catalyst. Such coated, packed bed, or monolithic catalyst systems are well known to those skilled in the art. The reactor tubes are also provided with a water gas shift catalyst 150, which is located downstream from the steam reforming catalyst 105. The tubes 103 communicate with an outlet plenum 107, which delivers the reformate product to an outlet port 108. The reactor tubes 103 pass through holes in one or more baffles 109. The baffles 109 are chorded to allow fluid to flow around the end of the baffle and along the tube axis through a percentage of the cross-sectional area of the shell. The direction of the chorded side alternates by one hundred and eighty degrees such that fluid is forced to flow substantially perpendicular to the long axis of the tubes 103.
The reactor has a cold air inlet 112 in a shell-side of a water gas shift section, as well as, a hot air outlet 113. Most of the shell-side air is prevented from bypassing the hot air outlet 113 by an unchorded baffle 114, which fits snugly against the shell assembly 110 inner wall. The reactor is further provided in the shell side of a steam reforming section with a hot combustion product inlet 115 and a cooled combustion product outlet 116. The reactor is also provided with an external burner assembly 118. An adiabatic water gas shift reactor 121 is appended to the outlet tube header 106. The reactor employs both baffles 109, as well as, extended heat exchange surfaces, such as a plurality of closely-spaced plate fins 120, on the outer walls of the reactor tubes 103. The fins 120 are attached to all of the reactor tubes 103 in the tube array.
It has been determined that in the example of catalytic water gas shift as taught in the patent to Lomax et al., at temperatures below 350° C. the reaction rate is very slow, while at temperatures above 400° C. the thermodynamically-limited extent of reaction is undesirably low. Worse yet, at temperatures above 450° C. an undesirable side reaction to create methane begins to occur at appreciable rates. Thus, the total preferred operating temperature gradient is less than 50° C., and a gradient above 100° C. is quite undesirable. In the patent to Lomax et al., the feed gas to the catalytic water gas shift reactor is cooled with air that is near room temperature. The cold air used for cooling can cause extremely low temperatures in the zones of the catalytic reactor adjacent to the air inlet. Experience has shown that local temperature gradients of over 200° C. routinely occur, thus causing a significant reduction in reactor performance.